A valve or a gate is provided in a fluid passage through which a fluid such as a water flows, and is opened and closed to thereby regulate a flow of the fluid. In general, the valve or the gate is automatically opened and closed by an actuator in which an electric motor, a hydraulic motor or the like is used as a drive power source. In a case where this actuator is used, a ratio of opening of the valve or the gate can be ordinarily utilized as a piece of information for the opening and closing control thereof, and the actuator is provided with a detector (i.e., a sensor) such as an encoder or the like to thereby detect the ratio of opening of the valve or the gate, whereby it is possible to confirm the ratio of opening of the valve or the gate at a place at a place around the actuator or a monitoring station where is remote from the valve or the gate.
An example of a prior art actuator in which the ratio of opening of the valve and so forth can be caught by using the encoder or the like is disclosed in JP-2004-257419 A and JP-2004-257420 A.